See the main article on sponges for a detailed introduction to sponge biology.
One of the central challenges in sponge biology is to understand the diversity of sponge architecture and form and the adaptations they confer on a particular sponge. We have already discussed how the diameter of supply, shown above, can potentially have a strong influence on teh internal architecture of the water canal or aquiferous architecture of the sponge body and on external form.
Sponges are filtration machines and suspension feeders: they filter food (chiefly bacteria and other fine organic particles) and oxygen from the water and their need to do this efficiently, whilst being sedentary and capable of only limited movements, dictates sponge form in large part. Other considerations are structural: sponges living in rough waters may form low encrusting structures that are not easily dislodged or broken by fierce currents or wave action.The need for the sponge to take in fresh water may result in intricate tree-like branching patterns which optimises diffusion across the still boundary layer. The system of canals and chambers through which water is pumped into, through and out of a sponge body is called the aquiferous system.
The sponges belong to the animal phlyum Porifera, so named because of
the many surface pores that take in water to be filtered by the sponge.
Classification schemes differ, but all generally recognise three main
divisions or classes (plus one or two others): the Calcarea
(calcisponges), the hexactinellids and the demosponges. This division is
based primarily on type of skeleton. The calcisponges have skeletons
made up of calcareous spicules (spicules are tiny spike-like structures
with precise architectures); the hexactinellids have spicules of
siliceous material and include the glass sponges and the demosponges
have siliceous spicules and/or spongin. In those demosponges lacking
spicules, the keratosa, fibrils of the spongy protein spongin form the
main skeleton and cover the sponge in a tough leathery membrane. Sponges
can also be classified by the internal architecture of their aquiferous
systems as ascons, sycons or leucons. Note that some sponges have mixed
or intermediate architectures but we can consider either the predominant
or most complex architecture present in any particular sponge body.
Now we shall look at a few examples of different sponges.
Leucosolenia is a genus of clacareous sponge of the class Calcarea (the calcisponges). It has the simplest form of aquiferous architecture: the ascon type, in which water flows in through tiny pores on the sponge body into the main internal chamber (spongocoel) which is lined by the choanocytes that do the water-pumping and then water exits through the main terminal exhalent pore or osculum.
The simplest possible arrangement of a single vase-like ascon, or olynthus, is not realised in adult sponges. Instead the olynthus stage (if at all present) puts out hollow tubular extensions which contain extensions of the central cavity or spongocoel and feed into the main tube bearing the osculum. This network of tubules may bud off new vase-like structures at intervals, each developing its own osculum. There are no simple and solitary ascon in adult sponges. The adult ascon is therefore an interconnected group of ascon modules. The internal cavity of the sponge is continuous through the tubes and connects all the vase-like buds together. Some would consider this to be a colony of individuals, but in some forms these 'individuals' fuse together in complex ways. it is perhaps better to view such a sponge as a modular organism, built from repeated aquiferous modules, each with its own osculum.
Above: Leucosolenia variabilis, redrawn from Minchin et al. 1900. New modules or upright oscular tubes bud from a prostrate branching and anastomosing connecting network of tubes.
The internal skeleton consists of triradiate (3-rayed) and quadriradiate (4-rayed) spicules in the body wall (often of more than one type and size) whilst the skeleton supporting the osculum consists of sagittal types of spicules (a bit like barbed arrow heads) with the posterior rays projecting away from the osculum and the lateral ways interlaced to form the rim. Some monoaxon spicules (i.e. with a single axis) may project to form a fringe around the osculum.
Individual ascons of Leucosolenia may be up to 10 to 15 cm tall. Other species of Leucosolenia forms may put out tubes or diverticula to form branching tree-like forms (e.g. Leucosolenia complicata). In some tubular Leucosolenia species tubular, the network of tubes may fuse to form an outer layer or pseudoderm with a few large pseduopores. In Clathrina the body consists of a network of tubes which ramify into one or more oscula. Such a cage-like structure is called a clathrus.
Above: Clathrina clathrus, by Parent Géry, wikimedia Commons
If an ascon puts out diverticula that are determinate in growth (i.e. only grow to a finite length) then a sycon architecture results, as in Sycon. The diverticula of Sycon grow out in a regular manner on all sides just below the osculum as the sponge increases in height forming a strobiloid. In most sycons, however, the diverticula fuse at their ends by secondary growths or bridges, resulting in a form which outwardly looks like a smooth vase with perforations. In effect the wall has thickened and the sponge contains a series of small pumping chambers which maintains the pumping pressure as the sponge body enlarges (ensuring a good filtration rate and a good diameter of supply). All typical syconoids belong to the calcarea and usually have teh form of single vases or clusters of vases.
Halichondria is a genus of demosponges with siliceous spicules.
All demosponges have the leuconoid architecture, which is the most
complex class of aquiferous architecture seen in sponges. It usually has
a flattened encrusting growth form (growing on rocky coastlines where
wave action and tidal currents pose a hazard). The Breadcrumb Sponge (Halichondrium
panicea) grows in large encrustations up to 20 cm across and has
many minute oscula, each resembling a crater in a volcanic cone. Its
color varies from white to orange to yellow to green to brown and it
grows on hard surfaces in the mid-shore down to quite deep water. It
occurs on Mediterranean, Atlantic, English Channel, North Sea and west
Above: Halichondria panicea, the Breadcrumb Sponge, by Minette Layne, wikimedia Commons
The spiculation of Halichondria is as follows: various sizes of siliceous oxeas are strewn throughout the tissues, parallel to the body surface. An oxea is a monaxon spicule (i.e. elongated along one axis) which is pointed at each end (with both ends growing outwards from a central point during synthesis) like a needle with a point on each end.
About 85% of extant species of sponge are demosponges. An example of a demosponge lacking spicules is Verongia aerophoba (= Aplysina aerophoba) found on rocks in shallow water on Mediterranean and Atlantic coasts, north to Biscay. This sponge slacks spicules but may contain embedded hard nodules.
Above: Verongia aerophoba (= Aplysina aerophoba), by Esculapio, Wikimedia Commons
Although individual species of sponge may frequently be difficult to distinguish, requiring a microscopical examination of the spicules, the total diversity of form in the group as a whole is immense. Sponges may be sessile or stalked, branched and tree-like or tubular, fan-shaped or barrel-shaped, vase-like or trumpet-shaped, flat and encrusting or tall and elaborate. Understanding the needs of water filtration and mechanical strength can go a long way to explaining these remarkably diverse forms.
Oscarella lobularis was formerly classed as a demosopnge but now belongs to a 4th class of sponges, the homoscleromorphs.
Above: Oscarella lobularis by Parent Géry, Wikimedia Commons
Sponges of this new class have neither spicules nor spongin. O. lobularis occurs in the northeast Atlantic Ocean and in the Mediterranean. It is an encrusting, soft and gelatinous sponge with a velvety texture. It grows attached to rocks, stones or large seaweeds. It's body is about 0.5 cm thick and up to 10 cm in diameter and is lobed. It is usually colored pink-brown-yellow but is occasionally blue-violet. Ostia occur on the sides of the lobes and an osculum of about 1 cm diameter occurs on top of each lobe.
Although lacking a specialised skeleton its tissues are collagenous and
collagenous basement membranes underlie the epidermal layers and the
Above: A diagrammatic section through a small piece of Oscarella. (Based on Ereskovsky et al. 2015 and Ereskovsky, 2006). The external surface or exopinacoderm (uppermost, blue) is lined by a single layer of exopinacocytes, each cell of which bears a single flagellum. Wtaer is shown entering the aquiferous system via an ostium (top). The aquiferous canals are lined by a single layer of endopinacocytes (green). One of many choanocyte chambers is shown (purple), the outlet or apopyle of which is guarded by apopylar cells (blue). The collagenous basement membranes are shown in gery. The gelatinous ground tissue or mesohyl (yellow) contains many vacuolate cells (orange) a few archeocytes (blue) and a few granular cells (elongated rose-colored cell) and many symbiotic bacteria (grey rods).
Oscarella has some unusual forms of asexual propagation: on overhangs
it drips or releases tear-drop shaped propagules and can also form
bubble-like buds which detach and float away as diaspores (dispersal
Article created: 9/5/2020
Article updated: 6/6/2020
Ereskovsky, A.V. 2006. A new species of Oscarella (Demospongiae: Plakinidae) from the Western Sea of Japan. Zootaxa 1376: 37-51.
Ereskovsky, A.V.; Borisenko, I.E.; Lapebie, P.; Gazave, E.; Tokina, D.B. and Borchiellini, C. 2015. Oscarella lobularis (Homoscleromorpha, Porifera) Regeneration: epithelial morphogenesis and metaplasia. PLOS one DOI:10.1371/journal.pone.0134566.
Fry, W.G. 1979. Taxonomy, the individual and the sponge, in Biology and Systematics of Colonial Organisms, Larwood, G. and Rosen, B.R. (eds.). Academic Press.
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